The present invention relates generally to transducers and more specifically to an improved two wire transducer.
A transducer in a two wire system receives its power from the two wire system and transmits back a signal indicative of the physical parameter to be monitored. The power supply is at the remote location. The transducer module itself usually includes a transducer, an amplifier and a pair of terminals. The power is in the form of a DC current usually four milliamps, supplied over the pair of wires. The amplifier will bias its output to a DC potential which provides a supply of power for itself. With zero output voltage, there would be zero power available. Excess power not used by the amplifier is sunk to ground.
Typical prior art modules are illustrated in FIGS. 1 and 2 as including a transducer 10 connected to the pair of terminals 12 and 14 by a first impedance converter Q1 and an output driver or buffer. In FIG. 1, the output driver or current buffer is shown as a transistor Q2. In FIG. 2, the transistor Q2 is replaced by an operational amplifier A1. Bias control 16 has been added to control the output DC potential and hence provide a power supply for the device.
The demand exists for smart transducers or transducers which incorporate more system functions within the transducer module other than merely providing a signal on the output terminal 12. Kistler Instrument, Corporation, Assignee of the present patent, has heretofore used piezoelectric as the transducer 10 and has only been able to use a single stage amplifier. No other active devices or stages have been considered usable within the module to date.
Thus there exists the need to provide more stages in circuitry within the transducer module without degrading the signal quality.
Thus it is an object of the present invention to provide a smart transducer without increased size, power or wiring requirements.
Another object of the present invention is to provide a transducer module with an increased number of stages and circuitry without degrading the signal quality.
Still a further object of the present invention is to provide a piezoelectric transducer module operable in a two wire system, including additional stages and signal processing over that of the prior art.
These and other objects are achieved by providing a regulated power supply for the auxiliary circuits or stages using the available power in a transducer module. The regulated power supply may be a current source or current converted to a regulated voltage. The voltage required for the regulator is less than lowest transducer circuit output signal as controlled by the output or driver stage. The additional stages may be amplification or signal processing stages connected between the transducer and the output drive or buffer, and the auxiliary circuits provide other functions.
One auxiliary circuit is a self-testing circuit which is responsive to the power-up of the transducer for applying test signals to the transducer for a predetermined time. The tester would include an oscillator for generating test signals in response to a power-up and a disable circuit for disabling the oscillator after the predetermined time. This disabling is achieved by providing a reset signal to the oscillator after the expiration of the predetermined time as determined by a counter. The oscillator is maintained in the disabled state by a latch. Provisions also are provided to initialize a counter on a power-up.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.